In an optical fiber transmission system, the connection, including both line fiber sections and chromatic dispersion compensation fiber sections, connectors, and multiplexers/demultiplexers, together with couplers, filters, isolators, circulators, amplifiers, etc., behaves like a birefringent medium and induces effects that are harmful for signal propagation, giving rise in particular to variation in group delay time depending on the polarization angles of the light signals transmitted relative to the fast and slow axes of local birefringence, as shown in FIG. 1. In other words, as shown in FIG. 2, a propagation time difference, known as the differential group delay (DGD), appears between signals that are polarized along the fast axis and signals polarized along the slow axis. This delay t2−t1 written τ1, depends on the number of birefringent elements that have been passed through, and on the wavelength used.
As can also be seen in FIG. 2, this propagation time dispersion has the consequence of broadening the resulting light pulses after it has been conveyed along the optical connection, which broadening must be kept within a tolerance range specified by a value DGDmax that is determined as a function of the data rate, the coding, and the modulation format of the signal. DGDmax must be less than ½ where D is the data rate in bits per second (bit/s) of the light pulses transmitted over the connection.
DGD is essentially an instantaneous magnitude, since it depends on numerous physical factors that can vary over time, such as temperature, local applied stresses, etc.
Furthermore, the total dispersion due to the polarization of the light and to the birefringence of the medium can also be characterized by another magnitude known as polarization mode dispersion (PMD) that takes account of the average of the DGDs for all of the polarization states and for the entire optical spectrum conveyed by the fiber over the duration of the PMD measurement. It has been shown that the various DGD values that are obtained over a broad spectrum range and over a short duration, e.g. over a few minutes, correspond to the values that the DGD can take at a given wavelength over a longer period of time, e.g. of the order of several days. This broadband optical measurement can be performed if, and only if, a significant spectrum range is explored, and consequently, the spectrum range that can be covered by the passband of a channel in a wavelength division multiplexing (WDM) system, as set by the optical multiplexers/demultiplexers of the terminals or by the optical add drop multiplexers (OADMs) is found to be too small to enable such a measurement to be performed.
It is possible, in known manner, to estimate a maximum differential group delay value DGDmax from the measured PMD by calculating the probability of a DGD value being exceeded as a function of a statistical model of DGD distribution, such as the now-standardized Maxwell distribution.
A known method of measuring PMD over an optical fiber connection consists in measuring PMD on fiber sections of the connection by means of a broadband light source or a light source that is turnable over a broad band, e.g. a band of several tens of nanometers. For this purpose, it is possible to make use of the entire optical passband of a WDM connection. A polarization controller makes it possible to simulate all of the polarization states of the signal at the inlets of fiber sections. At the outlets from the sections, a second controller serves to analyze all of the polarization states of the received signal. The PMD value is estimated from interferograms of the received signal or from an analysis of polarization states, implementing a Jones matrix, Stokes parameters, or the Poincaré sphere, for each wavelength and for each analyzed polarization state. Probabilistic processing covering all of the polarization states and all of the wavelengths gives an estimate for the PMD value, and thus an estimate for DGDmax by applying the above-mentioned statistical model.
That known method of estimating DGD nevertheless presents a certain number of drawbacks. Firstly, it does not enable an instantaneous value of DGD to be obtained, even though that is of very great importance when it is desired to qualify a connection for acceptance purposes. Furthermore, it constitutes a method that is intrusive, causing traffic to be interrupted since it requires the optical line to be interrupted in order to insert the measurement equipment. Finally, only optical fiber sections are taken into account rather than all of the elements in the connection, and in particular the terminals and the OADMs are not taken into account.